1. Field of the Invention
The present invention relates to a method for analyzing impurities in a silicon substrate represented by a silicon wafer, and an apparatus for decomposing a silicon substrate through vapor-phase reaction. More specifically, the present invention relates to a method for analyzing impurities in a silicon substrate without resorting to heating or pressurizing, and an apparatus for decomposing a silicon substrate through vapor-phase reaction.
2. Description of the Related Art
Metallic impurities contained in a silicon wafer cause leak current of the device to increase and the oxide membrane to degrade. Therefore, as the semiconductor device is more densely integrated to perform a higher function, impurities thereof come to have a greater impact on the performance of the device. To meet this situation, demand becomes acute for a method by which it is possible to directly evaluate the purity of a silicon wafer by dissolving the wafer itself with an acid and then analyzing the residue, instead of resorting to indirect assessment based on the measurement of electric properties of the wafer.
The known method for dissolving a silicon wafer includes direct and indirect methods: the former method (for example, M. Shabani et al., Proceedings of the Autumnal Meeting of the Japanese Society for Analytical Chemistry, 2H 09, p405 (1995)) consists of admixing a silicon wafer with an acid mixture of hydrofluoric acid and nitric acid so as to dissolve the wafer, and the latter method of vaporizing an acid mixture of hydrofluoric acid and nitric acid and applying the resulting gas to a wafer for decomposition. The indirect method is further classified into a method of pressurized decomposition and a method of non-pressurized decomposition.
Because the method of non-pressurized decomposition requires, for decomposition, a period as long as about five to ten days, and has been regarded as impractical, interest has been directed towards the method of pressurized decomposition. As such the method of pressurized decomposition, for example, Japanese Patent Laid-Open No. Hei 7-333121 (333121/1995) discloses a treatment method for high precision analysis of the impurities in a siliceous sample. The method consists of separately storing an analysis sample container in which a siliceous analysis sample is mounted and a decomposing solution comprising an acid mixture of hydrofluoric acid and nitric acid in a sealed vessel in such a way as to prevent the analysis sample from directly contacting with the solution; heating the vessel to cause the analysis sample to decompose into a sublimate; and recovering the residue in the analysis sample container.
This treatment method not only reduces the time necessary for analysis, but allows only a gas vaporized from the decomposition solution to contact with the analysis sample, and to decompose the analysis sample into a sublimate, because with this method the analysis sample and the decomposition solution are placed separately from each other in the vessel to avoid direct contact.
The above method based on pressurized decomposition, however, poses a number of problems: for the siliceous analysis sample to be safely decomposed into a sublimate in an atmosphere pressurized and heated, the vessel must be highly tightly sealed, and thus have walls with a considerable thickness of about 10-20 mm, particularly when the vessel is made of polytetrafluoroethylene (PTFE or Teflon(TM)). Moreover, because a large number of dew drops develop on the ceiling surface of the vessel, the vessel must have a dome-shaped ceiling in order to prevent dew drops from falling upon the sample.